A fiber-optic pH sensor and associated instrumentation were developed for measurement and monitoring of intraesophageal and intragastric pH. Composed of replaceable sensors, an optical system, and electronic circuitry, the instrument measures pH in the range of 0-7, with an accuracy of 0.1 pH units and a response time in the range of 30 sec, for up to 24 hours. This instrument should be useful for the research, diagnosis, and evaluation of acid-related disorders of the upper gastrointestinal tract, and for studies on the influence of diet and antisecretory drugs.